Phenolic resins are used in various fields as binders of either organic or inorganic substances because they are excellent in their performance and working ability. Typical instances are foundry binders such as used for coating phenolic resins onto sand particles, and the binder for molding material, for incorporating fillers such as wood flour, pulp, glass fiber or inorganic substances into a phenolic resin. In the process of hot mulling in foundry applications, and that of kneading by hot roll in molding material production, phenolic resin is required to be solid in ambient temperature for processing. Therefore, both novolac type resin and solid resole type resin are used for this purpose.
Since a novolac type resin does not cure by itself by heating, generally a curing agent such as hexamethylene tetramine (hereinafter called "hexamine") is incorporated into said resin. However, when hexamine is incorporated as a curing agent, there is a drawback of contaminating the air of the work site, emitting toxic and disagreeable gases such as ammonia or amines by the crosslinking reaction of novolac type resin. When novolac type resin is used as a binder for molding material, ammonia or amines remaining in the cured article corrodes inserted metals, and this causes a drawback comprising an abnormal leakage of electricity and fracture of molded parts of mechanical equipment. On the contrary, resole type resins have a self-curing property when heated, have an advantage of being free of such emission of ammonia or amines at its curing or said damage of products. Accordingly, solid resole type resins have recently had several uses such as for foundries, refractories, molding material an so forth. However, solid resole type phenolic resin has the general drawback of slow rate of cure compared with novolac type resin cured with hexamine. The adjustment of the hexamine ratio to novolac type resin is generally easy with respect to the control of its degree of condensation. Also, hexamine is fast in its rate of cure of novolac type resin by heating. Compared with this, solid resole type resins must generally be kept lower in their degree of condensation so as to prevent gel formation in the reaction kettle during their preparation. Also, they are essentially slower in rate of cure than the novolac type resin with hexamine. To accelerate the rate of cure in the solid resole type phenolic resin, several additives are reported to be effective; hydroxide or oxide of magnesium, zinc or barium, bisphenol S or catechol (Japanese Patent Publication Sho No. 53-58430), reactive phenols such as resorcinol, or acids such as salicylic acid (Japanese Patent Publication Sho No. 54-28357). While these additives can accelerate the rate of cure in the initial and medium period for solid resole type resin, they cannot accelerate it at the terminating period (in practical applications). Heating solid resole type resin with these additives incorporated therein seems apparently to accelerate the cure of resin due to reaction with said additives after melting of the resin. A tighter crosslinking structure cannot be achieved by further heating of the resin composition since initial formation of a coarse crosslinking interrupts free movement of molecules necessary for the curing reaction thereafter, thereby reducing the opportunity for contact of functional groups.
Assuming that said resin composition at its terminating period of cure is lower in crosslinking density than said solid resole type phenolic resin alone, this explains the phenomenon that the accelerating rate of cure with said additives in solid resole type phenolic resin causes a drawback in mechanical strength and hardness of finally cured articles.